Conventionally, in inverters which drive sensorless DC brushless motors with a PWM sinusoidal drive method, a method of detecting two currents out of the three output lines of the inverter is known (Japanese Patent Unexamined Publication No. 2000-333465, FIG. 2, page 9, for example).
The circuit for the above detecting method is described below. FIG. 11 shows the inverter and electrical circuits around it. To control circuit 7 of inverter 20, the phase U current is input from current sensor 8, and the phase W current is input from current sensor 9. Based on the values of these two currents, the phase V current is calculated by applying Kirchhoff's current law at the neutral point of stator winding 4. Using these phase U, phase V and phase W current values, induction voltage induced at stator winding 4 by magnet rotor 5 of sensorless DC brushless motor 11 (hereinafter referred to as motor) is calculated, for detecting the position of magnet rotor 5. A rotating speed-command signal (not shown), is used for controlling switching elements 2 formed of IGBT (Insulated Gate Bipolar Transistor), so that the direct-current voltage of battery 1 is switched by the PWM method. Accordingly, sine wave-like alternating current is outputted to stator winding 4 of motor 11. Diodes 3 of inverter circuit 10 provide a reflux route for a current which flows in stator winding 4. Switching elements 2 of the upper arm are defined as U, V and W, and those of the lower arm are defined as X, Y and Z. The diodes corresponding to respective switching elements U, V, W, X, Y and Z are defined as 3U, 3V, 3W, 3X, 3Y and 3Z.
Since electric potential changes to a plus side and to a minus side of battery 1, it is difficult to constitute current sensors 8 and 9 with shunt resistors; therefore, hall devices are used.
As another method for detecting the phase current, the method for detecting the phase current using a shunt resistor is disclosed, for example, in Japanese Patent Unexamined Publication No. 2003-189670 (page 2, claim 2, page 14, FIG. 1, page 15, FIG. 9).
In the following, the above detecting method is described. FIG. 12 shows the inverter and circuits around it. Control circuit 12 of inverter 21 calculates the current based on a voltage incurred at shunt resistor 6.
FIG. 13 is a waveform characteristics chart showing three phase modulation at 50% modulation, and FIG. 14 shows that at 100% modulation. Phase U terminal voltage 41, phase V terminal voltage 42, phase W terminal voltage 43, and voltage 29 at a neutral point is shown therein. Vertical axis represents a terminal voltage in terms of PWM duty (%). The voltage at neutral point 29 indicates the sum of terminal voltages of respective phases divided by three. The phase voltage value is the terminal voltage minus the neutral point voltage, which takes shape of a sine waveform.
FIG. 15 is a timing chart within one carrier (carrier cycle) of the three phase modulation. An exemplary ON/OFF status within one carrier (carrier cycle) of the upper arm switching elements U, V, W, and the lower arm switching elements X, Y, Z are shown. This is a timing chart in a case of the 50% modulation, approximate phase 120 degrees in FIG. 13. Generally, this is implemented by the timer function of a microcomputer. When the upper arm switching elements in the same phase are ON, those of the lower arm are OFF. On the contrary, when those of the upper arm are OFF, those of the lower arm are ON. In order to make the illustration simple, dead time for preventing the short-circuiting between the upper arm switching element and the lower arm switching element has been eliminated.
There are four states (a), (b), (c) and (d) in the switching of respective switching elements. FIG. 16 through FIG. 19 illustrate each of the above states.
In the period (a), all the upper arm switching elements U, V, W are OFF, whereas all the lower arm switching elements are ON. The phase U current and the phase V current flow to stator winding 4 from diode 3X and diode 3Y parallel to the lower arm switching elements X and Y, respectively, while the phase W current flows out of stator winding 4 to the lower arm switching element Z. A current is circulating between the lower arm and motor 11 (hereinafter referred to as lower circulating period). It is in the state of non-electric supply, during which there is no power supply from battery 1 to inverter circuit 10 (motor 11).
In the period (b), the upper arm switching element U is ON and the lower arm switching elements Y and Z are also ON. The phase U current flows from the upper arm switching element U to stator winding 4, and phase V current flows from the parallel diode 3Y parallel to lower switching element Y to stator winding 4. The phase W current flows out of stator winding 4 to the lower arm switching element Z. It is in the electric supply state, in which battery 1 supplies power to inverter circuit 10 (motor 11). At this time, the phase U current flows in the power source line (shunt resistor 6).
In the period (c), the upper arm switching elements U and V are ON, and the lower arm switching element Z is ON. The phase U current and the phase V current flow to stator winding 4 from the upper arm switching elements U and V, respectively, and the phase W current flows out of stator winding 4 to the lower arm switching element Z. It is in the electric supply state, in which battery 1 supplies power to inverter circuit 10 (motor 11). The phase W current flows in the power source line (shunt resistor 6).
In the period (d), all the upper arm switching elements U, V, W are ON, and all the lower arm switching elements X, Y, Z are OFF. The phase U current and phase V current flow from the upper switching element U and upper switching element V, respectively, to stator winding 4, and the phase W current flows out of stator winding 4 to the parallel diode 3W of the upper arm switching element W. A current is circulating between the upper arm and motor 11 (hereinafter referred to as upper circulating period). It is in the state of non-electric supply, in which no power is supplied from battery 1 to inverter circuit 10 (motor 11).
As described in the above, whether or not there is current, including the phase current, flowing in the power source line (shunt resistor 6) can be known from the ON/OFF state of the upper arm switching elements U, V, W. When there is no phase in which the upper arm switching element is ON, there is no current flow (non-electric supply, lower circulating period); when one phase is ON, there is current flow in that phase (electric supply); when two phases are ON, there is current flow in the remaining phase (electric supply); when all the three phases are ON, there is no current flow (non-electric supply, upper circulating period).
FIG. 20 illustrates, based on FIG. 15, the ON period (duty) of the upper arm switching elements U, V, W within one carrier (carrier cycle), in the 50% three phase modulation shown in FIG. 13, at phases 30 degrees, 45 degrees, 60 degrees, 75 degrees and 90 degrees.
ON period of the upper arm switching element U of phase U is represented by a fine solid line, ON period of the upper arm switching element V of phase V is represented by a medium solid line, and ON period of the upper arm switching element W of phase W is represented by a bold solid line. Period of electric supply, in which battery 1 supplies the power to stator winding 4, is shown with solid line arrow marks, and the phase currents then flowing in the power source line (shunt resistor 6) are designated with the indications U, V and W. Non-electric supply period (lower circulating period, upper circulating period) is shown with a broken line arrow marks.
FIG. 21 likewise illustrates the 100% three phase modulation of FIG. 14. As shown in FIG. 20 and FIG. 21, phase current can be detected by means of shunt resistor 6 for one phase or two phases. In a case where only one phase can be detected, a way of increasing, or decreasing, the ON period of parts of the phases (upper arm switching elements) has been disclosed (for example, in Japanese Patent Unexamined Publication No. 2003-189670, page 2, claim 2, page 14, FIG. 1, page 15, FIG. 9).
As shown in FIG. 20 and FIG. 21, the period (d) at the middle of carrier cycle in the three phase modulation is the non-electric supply period. In the two phase modulation, the period (d) does not exist since one phase is fixed. There is the non-electric supply period at the beginning and at the end within a carrier cycle. Therefore, there is an electric supply period in the first half and the latter half of a carrier cycle, respectively. As compared with the case of two phase modulation in which the electric supply (current flow) period is only one, the carrier cycle has been halved, namely the carrier frequency has been increased to be identical to double the frequency (hereinafter referred to as carrier cycle shortening effect), and the PWM modulation has been further elaborated. Thus, the current ripple and the torque ripple are reduced in the three phase modulation compared with two phase modulation. This reduces the vibration and the noise.
In the 100% modulation shown in FIG. 21, there is only one electric supply period in a carrier cycle at phase 30 degrees, hence, no carrier cycle shortening effect is made available. At phase 90 degrees, since there is no non-electric supply period at the beginning and at the end of a carrier cycle, the electric supply periods are mingled with those before and after the carrier cycle. So, even though there are two electric supply periods in a carrier cycle, eventual number of electric supply periods for each carrier cycle become to be only one, therefore, no carrier cycle shortening effect can be made available.
Still other method of detecting the phase current using shunt resistors is proposed (for example, Japanese Patent Unexamined Publication No. 2003-284374, page 7, FIG. 1).
The proposed method is described below. FIG. 22 shows the inverter and circuits around it. Control circuit 13 of inverter 22 calculates currents based on voltages respectively generated at shunt resistor 15 provided between the lower arm of phase U and the ground, at shunt resistor 16 provided between the lower arm of phase V and the ground, and at shunt resistor 17 provided between the lower arm of phase W and the ground.
FIG. 23 shows the ON period (duty) of lower arm switching elements X, Y, Z corresponding to FIG. 20. In order to make the illustration simple, the dead time for preventing the short-circuiting between the upper arm switching element and the lower arm switching element has been eliminated.
ON period of the lower arm switching element X of phase U is represented by a fine solid line, ON period of the lower arm switching element Y of phase V is represented by a medium solid line, and ON period of the lower arm switching element Z of phase W is represented by a bold solid lime. The lower circulating period is represented by bold broken line arrow marks, and the upper circulating period is represented by fine broken line arrow marks.
Likewise, FIG. 24 shows the ON period (duty) of lower arm switching elements X, Y, Z corresponding to FIG. 21. The time when current flows in shunt resistor 15, or the time when it can detect the current, is ON period of the lower arm switching element X. The time when current flows in shunt resistor 16, or the time when it can detect the current, is ON period of the lower arm switching element Y. The time when current flows in shunt resistor 17, or the time when it can detect the current, is ON period of the lower arm switching element Z.
Thus, in FIG. 24 the current can be detected only for the two phases (phase V, phase W) at phase 90 degrees, whereas, current can be detected with all the three phases in FIG. 23. Therefore, it has to be controlled so as to detect the current of phase W and phase U at phase 210 degrees, and phase U and phase V at phase 330 degrees.
As described in the above, in a three phase PWM inverter, there is a certain specific phase in which the electric supply period exists for one in a carrier cycle. Since there is no non-electric supply period at the beginning and at the end of a carrier cycle, the electric supply period is mingled with those before and after the carrier cycle. So, in a certain specific phase the carrier cycle, shortening effect can not be made available.
The conventional methods of detecting the phase current have their own respective problems to be solved.
A current sensor used in the method of detecting the current direct of output line from an inverter is formed of a hall device, a flux generating coil, the devices' peripheral circuit, etc. So, the size reduction and reinforcement of vibration-proof property are the problems to be solved. Furthermore, because of the bulky size, freedom degree of layout on a printed board is limited.
In the method of detecting a phase current which flows in the power source line by means of a shunt resistor; if the detection by shunt resistor is made for only one phase, it is required to increase, or decrease the ON period in parts of phases (upper arm switching element). This complicates software for controlling. Furthermore, increase or decrease of parts of phases (upper arm switching element) of the ON period brings about a distortion in the current waveform. This is an adverse factor against noise and vibration.
In a cases in which an air-conditioner compressor is driven with an inverter, it may be possible for the room air-conditioners to use a soundproofing device, such as a sound-proof box. In a case of a vehicle-mounted electric compressor, however, the space for installation and the allowable weight are limited. So, installation of a soundproofing device may be difficult. In addition, the intrusion of vibration into vehicle cabin has to be suppressed to a minimum, however, introduction of a quake isolating device may be almost as difficult by the same reasons. In the face of prevailing environment-conscious attitude, the noise and vibration due to room air-conditioners are also requested to be lower.
In the method of detecting a phase current by means of shunt resistors provided between the lower arm of each of phases and the ground, if there is a phase in which ON period of the lower arm switching element in a carrier cycle is 0% or close to 0%, it can be detected only for two certain specific phases. Therefore, the two certain specific phases to be detected need to be changed depending on phase. This complicates the control software. And, each of the three phases needs to be provided with a shunt resistor, which increases the parts counts. This is against the downsizing efforts, and power consumption and heat generation by the shunt resistors have to be considered too.
The present invention aims to solve the above-described conventional problems, and offers a compact and vibration-proof inverter which is capable of detecting a phase current without requiring the development of control software of high complexity and without causing any current distortion.